The present invention generally relates to processing a mask substrate. In particular, the present invention relates to dissipating electrostatic charges which accumulate during patterning of mask plates.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller features sizes are required. This includes the width and spacing of interconnecting lines and the surface geometry such as corners and edges of various features. Since numerous interconnecting lines are typically present on a semiconductor wafer, the trend toward higher device densities is a notable concern.
The requirement of small features (and close spacing between adjacent features) requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, X-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the photomask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating through the photomask causes a chemical transformation in the exposed areas of the coating thereby making the image area either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer.
Since the pattern image is generated by the photomask, providing a high quality photomask is critical to the formation of integrated circuits. Photomasks are often made of a patterned chromium layer over a quartz substrate. The patterned chromium layer is typically made using an electron beam resist. In particular, an electron beam resist is deposited and patterned over the chromium layer. The exposed portions of the chromium layer (not covered by the patterned electron beam resist) are etched and removed from the quartz substrate.
There are a number of problems associated with making photomasks. Patterning the electron beam resist of the mask plate structure involves irradiating the resist with an electron beam. The resist material is typically a hardened polymer material that is highly insulating. Electron beam irradiation undesirably produces electrostatic charge on the surface of the mask plate structure. Specifically, when an electron beam is directed at the insulating surface of the resist, electrostatic charge tends to accumulate on the surface of the resist, creating an electric field which may subsequently distort the electron beam adjacent the surface. This may lead to poor pattern resolution.
Another problem with making photomasks is inspecting the mask substrates with an electron beam emitting device. Using an electron beam, such as from a scanning electron microscope (SEM), to evaluate a patterned resist on a mask substrate tends to undesirably induce the formation of electrostatic charges on the surface of the resist. In light of these problems, there is an unmet need for preventing and/or dissipating electrostatic charge on photomasks during fabrication.
The present invention provides a system and a method for dissipating electrostatic charge which may accumulate on a patterned electron beam resist clad mask plate. More specifically, the present invention provides a system and method for removing the electrostatic charge formed, in part, during electron beam exposure and/or during inspection of a patterned electron beam resist clad mask plate.
One aspect of the present invention relates to a system for dissipating electrostatic charge, which has accumulated during patterning of mask plates, containing a mask substrate, a chromium layer deposited over the substrate, and a conductive electron beam photoresist deposited over the chromium layer; a conductive structure coupled to the mask plate structure which allows accumulated electrostatic charge to flow from the mask plate structure; a conductive path between the conductive structure and a ground, wherein the conductive path includes a switch controlled by a controller; and a detector coupled to the controller for signaling the controller when an accumulation of electrostatic charge is detected.
Another aspect of the present invention relates to a method for dissipating electrostatic charge, which has accumulated on a conductive electron beam resist during patterning of a mask plate, involving the steps of providing a mask substrate having a chromium layer; depositing a conductive electron beam resist over the chromium layer; connecting a conductive structure to the mask substrate; irradiating portions of the mask substrate with an electron beam; detecting whether electrostatic charge exists on the mask substrate; and if electrostatic charge is detected, closing a circuit whereby the conductive structure is grounded to permit a flow of electrostatic charge from the mask substrate to a ground.
Yet another aspect of the present invention relates to using a conductive polymer layer in a system for dissipating electrostatic charge existing on a mask plate. The system contains a mask plate structure comprising a mask substrate, a chromium layer deposited over the substrate, an electron beam resist deposited over the chromium layer, and a conductive polymer deposited over the electron beam resist for dissipating an accumulation of electrostatic charge; a conductive structure coupled to the mask plate structure; a conductive path running between the conductive structure and a ground, wherein the conductive path includes a switch controlled by a controller; and a detector coupled to the controller for signaling the controller when an accumulation of electrostatic charge is detected.
Still yet another aspect of the present invention relates to a method for dissipating electrostatic charge existing on a mask plate structure by employing a conductive polymer layer. The method involves the steps of providing a mask substrate having a chromium layer; depositing an electron beam resist over the chromium layer; depositing a conductive polymer layer over the electron beam resist; connecting a conductive structure to the mask substrate; irradiating portions of the mask substrate with an electron beam; detecting whether electrostatic charge exists on the mask substrate; and if electrostatic charge is detected, closing a circuit whereby the conductive structure is grounded to permit a flow of electrostatic charge from the mask substrate to a ground.